Epithelial tissue - definition, types, functions, examples (2023)

3. August 2023 go throughAnupama Sapkota

Table of contents

Definition of epithelial tissue

EpithelgewebeOne of four types of tissue (epithelial, muscular, connective, and nervous) in animals. It consists of densely packed polyhedral cells that adhere tightly to one another, forming lines that line hollow organs and cover layers of cells on the body's surface. Epithelial tissue or epithelial cells (plural epithelial cells) consist of one or more layers of cells arranged in a continuous layer.

Properties of epithelial tissue

Although epithelial tissues can vary in structure and function in different parts of the body, they all share some common characteristics.
Some of these characteristics are as follows:

shape and size

Epithelial cells vary in shape and size, from highly columnar to cuboid to low scaly.
The size and morphology of a cell often depends on its function.

polarity

Epithelial cells often exhibit polarity with an uneven intracellular distribution of organelles and membrane proteins.
The apical (free) surface of epithelial cells faces the body surface, body cavities, lumens of internal organs, or ducts that receive cell secretions. The apical surface may contain cilia or microvilli.
The sides of the epithelial cells face neighboring cells on either side and may contain cell-cell adhesions and other junctions.
The basal surface of epithelial cells is attached to extracellular material such as the basement membrane, an inert connective tissue formed by the epithelial cells themselves.

Basal membrane

The basement membrane is the thin outer layer of cells and is usually made up of two layers: the basement layer and the reticular layer.
The basal layer is closer to and secreted from the epithelium. It contains proteins like laminin and collagen, as well as some glycoproteins and proteoglycans.
Below that is the reticular layer, which is closer to the connective tissue and contains collagen produced by connective tissue cells called fibroblasts.

cell-cell adhesions and other connections

Several membrane-associated structures ensure adhesion and communication between cells.
Tight junctions, also called occluded zonules, are the apical junctions that form the band that completely surrounds each cell.
The second type of junction is the adherens junction or zonula adherens, which also surrounds the epithelium, usually just below the tight junction.
Another type of anchoring compound is the desmosome, or macula of adhesion, which are disk-like structures on the surface of one cell that conform to the same structure on the surface of a neighboring cell.
Gap junctions mediate intercellular communication rather than adhesion or occlusion between cells.

Avascular

Epithelial tissue is avascular and relies on the blood vessels of adjacent connective tissue to carry nutrients and remove waste.
The exchange of substances between epithelial tissue and connective tissue takes place by diffusion.

dominates

Epithelial tissue is innervated, which means it has its own nerve supply.

updates and fixes

Epithelial cells have a high rate of cell division, which allows the epithelial tissue to constantly renew and repair itself by shedding dead or injured cells and replacing them with new ones.

The function of the epithelial tissue

Depending on its location, epithelial tissue performs a number of functions. Some of them are:

Type/category with example and location

Epithelial tissue is divided into two types:

Covering and lining epithelium, also known as the superficial epithelium, forms the skin and outer covering of some internal organs, as well as the lining of blood vessels, ducts, body cavities, and the lining of the respiratory, digestive, urinary, and reproductive systems.
glandular epitheliumIt forms the secretory part of glands such as the thyroid, adrenal glands, sweat glands and digestive glands.

In addition, the types of covering and lining epithelial tissue are classified according to the arrangement of the cells and the shape of these cells.

Image source:Rice University (OpenStax)

simple epithelium

Simple epithelium consists of a single layer of identical cells normally located on secretory and absorbent surfaces, and the monolayer enhances these processes.
Individual epithelium is divided into three main types, named for the shape of the cells, which vary by function.

A. Simple squamous epithelium

Image source:Rice University (OpenStax)

Simple squamous epithelium consists of a single layer of flattened cells that, when viewed from the apical surface, resemble floor tiles with a centrally located flattened, oval, or spherical nucleus.
This epithelium is most commonly found in cardiovascular and cardiovascular diseaseslymphatic system（Herz, blood vessels, lymph vessels), it is saidendothelial cellsand forms the epithelial layer called the serosa (peritoneum, pleura, pericardium).
It is also found in the air sacs of the lungs, the glomerular (Bowman's) capsule of the kidneys, and the inner surface of the eardrum (tympanic membrane).

b. Simple cuboidal epithelium

Image source:Rice University (OpenStax)

Simple cuboidal epithelium is a single layer of cuboid cells with a rounded shape and a centrally located nucleus.
It covers the surface of the ovary, lines the anterior surface of the lens capsule of the eye, forms the pigment epithelium on the posterior surface of the retina of the eye, lines the renal tubules and the smaller ducts of various glands, and forms the secretory part of some glands (e.g . the thyroid gland) and some glandular ducts, e.g. B. the pancreas.

C. Simple columnar epithelium

Image source:Rice University (OpenStax)

Columnar epithelium consists of a single layer of rectangular cells on a basement membrane.
This epithelium lines many organs and is often derived to make it well suited for specific functions.
Columnar epithelium lines the stomach with no surface structure. However, the free surface of the columnar epithelium lining the small intestine is covered with microvilli, which provide a large surface area for absorption of nutrients from the small intestine.
In the trachea, the columnar epithelium is ciliated. In addition, it contains mucus-secreting goblet cells, and in the fallopian tube, the ovum is carried toward the uterus by the action of cilia.

stratified epithelium

Stratified epithelium consists of several cell layers of different shapes and usually lacks a basement membrane.
When the basal cells divide, the resulting daughter cells push the old cells up into the top layer.
As they move to the surface and move away from the blood supply to the underlying connective tissue, they become dehydrated and less metabolically active.
When the cytoplasm is reduced and tough proteins dominate, the cell becomes a tough, rigid structure and eventually dies.
In the top layer, dead cells are shed when they lose their cell connections. However, they are constantly being replaced as new cells emerge from the basal cells.
There are two main types of stratified epithelium: stratified squamous epithelium, stratified cuboidal epithelium, and stratified columnar epithelium.

A. Stratified squamous epithelium

Image source:Rice University (OpenStax)

Stratified squamous epithelium consists of two or more layers of cells. The cells of the top layer and the lower layers are scale-shaped, while the cells of the lower layers vary from parallelepipedal to columnar.

I. Keratinized stratified squamous epithelium

This epithelium forms a layer of tough keratin at the top of the cells and several layers deep inside
When a cell is deprived of the blood supply of nutrients, the relative amount of keratin in the cell increases and the organelle eventually dies.
Keratin forms a tough, relatively waterproof protective layer that protects the living cells underneath from drying out.
Keratinized stratified squamous epithelium forms the surface layer of the skin.

2. Non-keratinizing stratified squamous epithelium

This epithelium does not contain much keratin in the top layer and several deeper layers and is constantly moistened with mucus from the salivary and mucous glands.
Non-keratinized stratified squamous epithelium covers the moist surfaces (lining of the oral cavity, esophagus, parts of the epiglottis, parts of the pharynx and vagina) and covers the tongue.

b. Stratified cuboidal epithelium

Image source:Rice University (OpenStax)

Stratified cuboid epithelium consists of multiple layers of cells, with the apical layer consisting of cuboidal cells while the deeper layers may be cuboid or columnar.
In the ducts of the salivary and sweat glands there is stratified cuboidal epithelium.

C. Stratified columnar epithelium

Image source:Rice University (OpenStax)

Stratified columnar epithelium consists of multiple layers of cells, with the apical layer consisting of columnar cells while the deeper layers can be cuboid or columnar.
This type of epithelium is found in small areas of the eye's conjunctiva, parts of the urethra, and anal mucosa.

pseudostratified columnar epithelium

Image source:Rice University (OpenStax)

Pseudostratified epithelium appears to be stratified as nuclei are present at different levels.
Although all cells adhered to the basement membrane in a monolayer, some cells did not reach the apical surface.
Because of these features, it looks like layered tissue, but is actually simple epithelium.
This epithelium lines the epididymis, the major ducts of many glands, as well as part of the male urethra and most of the upper airway.

transitional epithelium

Figure: Transitional epithelial tissue. Image source:Wikipedia。

The transitional epithelium (urothelium) has a different appearance (transition).
In the relaxed or unstretched state, it looks like stratified cuboid epithelium, but the apical cells tend to be broader and rounded.
When the tissue is stretched, the cells flatten out and take on the appearance of a stratified squamous epithelium. The layers and elasticity make it ideal for lining hollow structures (bladders) that are easily inflated from the inside.

glandular epithelium

Epithelial cells whose main function is to produce and secrete various macromolecules may be present in epithelial cells with other important functions or may contain specialized organs called glands.
Scattered secretory cells, sometimes called unicellular glands, are commonly found in simple cuboid, simple columnar, and pseudostratified epithelium.
The gland develops from the epithelium covered by the fetus by cell proliferation and growth into the underlying connective tissue, which then further differentiates.

Figure: Main types of glands. During embryonic development, glands differentiate from epithelial tissue. Arrows indicate released substances. Image source:University of Vigo。

endocrine gland

Secretions of the endocrine glands, called hormones, get into the interstitial fluid and then diffuse into the blood without flowing through the ducts.
Endocrine secretions have profound effects as they are distributed throughout the body via the blood.
Examples of endocrine glands include the pituitary gland at the base of the brain, the pineal gland in the brain, the thyroid and parathyroid glands near the voice box (larynx), the adrenal glands above the kidneys, the pancreas near the stomach, and the ovaries in the Pelvis, the testicles and the thymus gland in the scrotum. The thymus gland is located in the chest cavity.

exocrine glands

Eccrine glands secrete their products into ducts that deliver the secretions to organ surfaces, such as the skin surface or the lumen of a hollow organ.
The effects of eccrine gland secretions are limited and some of these can be harmful if they enter the bloodstream.
Digestive glands such as the sweat, oil and cerumen glands of the skin, the salivary glands (secreting into the mouth), and the pancreas (secreting into the small intestine) are examples of exocrine glands.

Reference

Mescher, AL (2016). Basic histology. Fourteenth edition. McGraw-Hill training.
Tortola GJ and Derrickson B (2017). Principles of physiology and anatomy. Fifteenth edition. John Wiley & Sons
Waugh A und Grant A. (2004) Anatomy and Physiology. neunte Auflage. Churchill Livingston.
https://www.kenhub.com/en/library/anatomy/overview-and-types-of-epithelial-tissue

internet resources

3 % – https://quizlet.com/25768290/chapter-3-tissues-flash-cards/
2 % – https://quizlet.com/47458913/chapter-4-flash-cards/
2 % – https://quizlet.com/26692499/chapt-4-epithelial-flash-cards/
2% – https://quizlet.com/25669314/anatomy-ch-4-reading-flash-cards/
2 % – https://doctor2017.jumedicine.com/wp-content/uploads/sites/7/2018/01/Surface-epithelium.pdf
1 % – https://www.youtube.com/watch?v=0NEV-Rd7OgA
1 % – https://www.thoughtco.com/animal-anatomy-apphire-tissue-373206
1 % – https://www.thenursepage.com/applipi-tissues-types-of-apphire-tissues/
1 % – https://www.cram.com/flashcards/ap-i-ch4-6264721
1 % – https://www.coursehero.com/file/p5d8jsc/Epithelial-tissue-may-be-divided-into-two-types-1-Covering-and-lined/
1 % – https://study.com/academy/lesson/simple-cuboidal-epithelium-location-struct-function.html
1 % – https://quizlet.com/gb/340412792/histology-epithelia-flash-cards/
1 % – https://quizlet.com/ca/310892532/chapter-4-bio-235-flash-cards/
1 % – https://quizlet.com/95543301/apppithel-tissue-covering-and-lined-epithelium-flash-cards/
1 % – https://quizlet.com/72533810/anatomy-and-physiology-tissue-level-of-organization-flash-cards/
1 % – https://quizlet.com/6320460/anatom-ch3-terms2-flash-cards/
1 % – https://quizlet.com/50190298/44-epithelium-quiz-1-flash-cards/
1 % – https://quizlet.com/326765543/apchapter-4-the-tissue-level-of-organization-flash-cards/
1 % – https://quizlet.com/26142932/anatomy-lecture-3-tissues-flash-cards/
1 % – https://quizlet.com/25583191/anatomy-ch-4-epithelial-tissue-flash-cards/
1 % – https://quizlet.com/225671385/final-exam-special-objectives-chapter-4-tissues-flash-cards/
1 % – https://quizlet.com/211826315/chapter-4-the-tissue-level-of-organization-flash-cards/
1 % – https://quizlet.com/20156045/the-tissue-level-of-organization-flash-cards/
1 % – https://quizlet.com/17285188/biomed-module-7-flash-cards/
1 % – https://quizlet.com/14302876/epithelium-flash-cards/
1 % – https://quizlet.com/13823304/classification-of-tissues-part-1-flash-cards/
1 % – https://healthjade.net/basement-membrane/
1 % – https://doctor2019.jumedicine.com/wp-content/uploads/sites/10/2020/01/Glandular-Epithelium.pdf.pdf
<1% – https://www.studymode.com/essays/Outline-The-Structure-Of-The-Main-1455732.html
<1 % – https://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/epithel.htm
<1 % – https://www.kenhub.com/en/library/anatomy/overview-and-types-of-epithelium
<1 % – https://www.difference Between.com/difference- Between-simple-stratified-and-pseudostratified-apphire-tissue/
<1% – https://www.coursehero.com/file/p502iic/630x-LM-Mark-Nielsen-Sectional-view-of-keratinized-stratified-squamous/
<1 % – https://www.britannica.com/science/microvilli
<1% – https://quizlet.com/99144165/epithelium-tissue-functions-and-locations-flash-cards/
<1% – https://quizlet.com/9814963/chapter-5-tissues-flash-cards/
<1% – https://quizlet.com/24665603/body-tissues-flash-cards/
<1% – https://quizlet.com/18641983/chapter-3-cells-and-tissues-flash-cards/
<1 % – https://oerpub.github.io/epubjs-demo-book/content/m46048.xhtml
<1 % – https://micro.magnet.fsu.edu/primer/anatomy/brightfieldgallery/index.html
<1 % – https://en.wikipedia.org/wiki/Stratified_cuboidal_epithelium
<1% – https://en.m.wikipedia.org/wiki/Urethra
<1% – https://courses.lumenlearning.com/boundless-ap/chapter/apppire-tissue/
<1 % – https://courses.lumenlearning.com/boundless-ap/chapter/absorbation/
<1% – https://anatomyandphysiologyi.com/apppire-tissue/
<1 % – http://www.histology.leeds.ac.uk/tissue_types/epithelia/epi_specializations.php
<1 % – http://www.difference Between.net/science/health/difference- Between-exocrine-and-endocrine/
<1 % – http://medcell.med.yale.edu/systems_cell_biology/epithelium_lab.php

Epithelial tissue - definition, types, functions, examples (2023)

Definition of epithelial tissue

Properties of epithelial tissue

The function of the epithelial tissue

Type/category with example and location

simple epithelium

A. Simple squamous epithelium

b. Simple cuboidal epithelium

C. Simple columnar epithelium

stratified epithelium

A. Stratified squamous epithelium

b. Stratified cuboidal epithelium

C. Stratified columnar epithelium

pseudostratified columnar epithelium

transitional epithelium

glandular epithelium

endocrine gland

exocrine glands

Reference

internet resources

References